The finish on construction lumber, such as cut from logs into 2×4, 2×6, 2×12, and 4×4 nominal dimensions, has become increasingly important as a result of the trend for such construction lumber to be sold in retail outlets, such as the large home improvement chain stores, to “do-it-yourself” (DIY) consumers. While construction lumber is typically covered with sheet rock or gypsum board and so is not visible in finished construction, DIY consumers often select and purchase construction lumber primarily on the basis of surface finish. Accordingly, well finished construction lumber can command a premium price, and construction lumber that is not well finished may be difficult to sell.
Commercial planer heads include a plurality of elongate knives spaced circumferentially on a cylindrical hub rotating at high speed. The elongate axes of the knives are typically, but not necessarily, aligned with the axis of rotation. The wood travels relative to the head in a direction perpendicular to the axis of rotation of the hub, the knives cutting a surface on the wood. The resulting surface finish is affected by a number of factors, e.g., the extent to which the planer head is in balance, the density of knives on the planer head, the speed of rotation, the speed of travel of the wood, and the ability of the apparatus to efficiently keep chips away from the cutting surface as it is being cut.
Standard practice provides for statically balancing the knives and knife assemblies carried by the hub as well as dynamically balancing the hub and the shaft to which the hub is attached (or with which the hub is integrally formed). The speed of rotation of the head is set as high as practical, and the speed of travel of the wood is set as high as possible while still providing acceptable surface finish, to increase the speed of production. Some “chip marks” occur as a result of chips remaining on the cutting surface as the wood is being cut and have been accepted in the prior art.
With a given degree of balancing and speed of rotation of the planer head, increasing the speed of wood travel to obtain further efficiency increases will decrease the quality of the surface finish, and it would be advantageous either to be able to increase the speed while maintaining the quality of surface finish, or maintain the speed and improve the quality of the surface finish.
One means for increasing the surface quality given the limitations noted above is to increase the frequency of cutting by increasing the density of knives on the planer head. Particularly, the parallel and circumferentially distributed cutting edges of the knives should be spaced as close together as possible. However, each knife must be removable so that the knife can be sharpened or replaced. The knives are typically clamped in knife assemblies by screws. The screws may bear on a block of metal called a “gib” that it turn bears on the knife, the end of the screw may bear directly on the knife, or the screw may extend into a collar that wedges against the knife. In all cases, the construction methodology places limits on the potential for increasing the density of the knives.
Typically, prior art knives have a straight cutting edge and one or two radiused or semi-circularly curved cutting edges at respective ends of the straight cutting edge. Where only one curved cutting edge is employed, the knives are alternated in upside-down and right-side-up position so that two knives together cut respective opposite corners of the wood and each knife cuts the straight face of the wood so that the straight face of the wood is twice cut. In either case, a straight cutting edge is physically merged with a curved cutting edge. In a process known as “jointing,” used for sharpening the knives as the knives are installed in the planer head and as the head is rotating, wherein a fixed stone is introduced against the rotating knives, what is known in the art as “relief” is lost for the outer portions of the curved cutting edges. This lack of relief results in hammering the wood at the corners, degrading surface finish.
Another problem in the prior art is adjusting the planer head between cutting an article of wood from green wood stock to cutting an article of the same nominal size from dry wood stock, and vice-versa. This has required replacing the complete planer head, which is costly.
There is a need, therefore, for a high speed planer head provided according to the present invention that solves the aforementioned problems and provides additional features and advantages.